The present invention relates to a computed tomographic imaging device and a method for operating such a device in a continuous or helical mode with automatic adjustment of the device based on analysis of the data being gathered to later construct images. Computed Tomography (also called Computed Axial Tomography, CAT, or CT scanning) has evolved through several major changes, or generations, in its roughly 25 year history. The earliest scanners collimated the x-rays from a tube source into a "pencil beam" and scanned that beam linearly across the object or patient to be imaged and onto a single, synchronously moving detector, or later onto a linear array of detectors. These measurements comprised one view of the object. The tube and detector assembly (usually mounted on a rotating gantry) were then rotated a small angle around the object and another view was acquired. The steps of linear motion and rotation in a plane were repeated many times over at least 180 degrees, usually over a full circle or 360 degrees. Given enough view data in a single plane, any of several known algorithms can be used to reconstruct the planar representation of local x-ray attenuation, that is, a CT image.
A major improvement in CT occurred with adoption of so-called fan beam technology. In this advance, the x-ray beam is collimated into a nearly planar beam, tightly collimated in the axial or z-direction defining the thickness of a slice to be imaged, but spread out over an angle, called the fan angle, to cover the complete width of the object and a generally curved array of detectors on the opposite side of the object. In so-called third-generation CT designs, the detector array is usually curved, is roughly matched in azimuthal extent to the fan of x-rays, and rotates on the gantry to remain opposite the imaged object. In fourth-generation designs, the detector array covers the full arc of view data, generally a full circle, and is fixed so only a varying portion is irradiated by the x-ray fan beam as it rotates. As initially introduced, these devices acquired the full set of azimuthal views for one slice before moving the object (generally on an axially movable table) to a new slice location and acquiring data for a new planar image. For this step-and-shoot operation planar reconstruction methods are substantially correct.
The next major geometry change in CT was development of methods for helical scanning. In this advance, the scanner hardware geometry is largely unchanged--the x-ray tube and the curvilinear detector array rotate in a plane around the imaged object which is substantially perpendicular to its axis. However, now during the rotation the object is moved along the axis (usually on a moving table), so the net path of both tube and detector around the object is a helix. The axial distance covered during one gantry rotation divided by the slice thickness is called the helical pitch. Because the inter-scan delay for advancing the object is eliminated, helical scanning can cover more axial distance in a given time than the earlier step-and-shoot methods. However, because the view data for an image are no longer coplanar, image reconstruction methods must be modified to reduce the potential for artifacts. Helical scanning was introduced to commercial medical CT in the late 1980s and dominated clinical scanning protocols by the mid-1990s.
The latest major change in CT is introduction of two-dimensional or multi-row detector arrays. In this advance, the single arced row of detector cells is replaced by an array which is segmented in both the usual azimuthal direction and the axial direction. A fourth-generation scanner operating with a three-row detector is described by Heuscher et. al. in U.S. Pat. No. 4,965,726. A third-generation scanner with a two-row detector is described by Arenson et. al. in U.S. Pat. No. 5,228,069. In U.S. Pat. No. 5,291,402, Pfoh describes a general approach to multi-slice scanning in which the detector can be arranged to have any number of rows. Since these detectors can cover several slices in the axial dimension, collimation of the x-rays at the tube is opened up to cover substantially the full array; the beam has an axial (or cone) angle in addition to its azimuthal (or fan) angle. Except for the simplest case of a butted two-row detector, that of Arenson et. al. and essentially that of Heuscher et. al., the slice thickness or axial resolution then becomes determined at the detector rather than at the x-ray tube collimator. Reconstruction algorithms now must deal with data gathered from more than one row of detectors, and, of course, they are designed to produce more than one image of the object per system rotation. In addition, since the planes defined by the separate detector rows are not coplanar, further changes must be made to combat artifacts from non-coplanar data. Many special reconstruction algorithms have been developed for such helical, non-coplanar scanning, for example those described by Hu in U.S. Pat. Nos. 5,377,250, 5,400,255, 5,430,783, 5,513,236, and 5,541,970, by Tam in U.S. Pat. Nos. 5,390,112 and 5,504,792, and by Saito in U.S. Pat. No. 5,541,971.
The combination of multi-slice detectors with helical scanning compounds the challenge to dealing with non-coplanarity or incompleteness in the view data, but for small enough cone angles and small enough scan pitches these problems have been solved and this combination appears to be the technology of choice for high-end CT systems.
CT scanners today generally operate with preselected scan protocols. That is, before the start of the scan, the operator selects the volume to be covered (e.g. diameter of field of view and axial coverage) and several machine parameters (e.g. rotation speed, x-ray tube voltage, x-ray tube current, slice thickness, helical pitch, and mathematical parameters of the reconstruction algorithm, such as those which determine view filtering prior to backprojection). When these choices are made, the scan is started and proceeds to completion without change of protocol.
In some cases, large temporal changes occur in the object during the course of the scan. For example, in scans of patient cardiovascular anatomy the motion of the heart and blood or the bolus action of contrast agents injected in the vascular system produce such changes. Because standard CT reconstructions presume consistency of all views used, changes in the object during acquisition of view data will produce blurring and other significant artifacts. Several methods have been developed to deal with such temporal changes, among them those by Yamagishi in U.S. Pat. No. 5,383,231, Brown in U.S. Pat. No. 5,459,769, Toki et. al. in U.S. Pat. No. 5,612,985, Bae et. al. in U.S. Pat. No. 5,687,208, and Lutz in U.S. Pat. No. 5,832,051.
An object which is to be imaged is also typically not cylindrically uniform in space. Methods have been developed to determine spatial variations of the object in advance of a CT scan--for example, from a planar projection image called variously a scoutview or a scanogram--then predetermine and preprogram scan parameter changes which optimally account for this spatial structure. For example, Toth in U.S. Pat. No. 5,379,333 teaches a method for adjusting x-ray flux during gantry rotation around objects with high aspect ratio, such as human shoulders. Fujimoto et. al. in U.S. Pat. No. 5,386,446 teaches a method for predetermining axial regions of the object which would be best scanned with higher or lower image resolution, then preprogramming and controlling the scanner to do so in one scan operation.
In addition, during the course of a continuous or helical scan new regions of the scanned object continually enter, then progressively pass through, an imaging volume--the volume which at all times during the scan is being traversed with x-rays from the source to the detector. The imaging volume is generally a cylinder with center line on the bore of the scanner, with diameter which shadows the useful azimuthal extent of the detector, and with axial length equal to the axial x-ray beam width or that which shadows the useful axial extent of the detector, whichever is smaller. The passage of a given slice region of the scanned object across the imaging volume (i.e., the time when it is affecting the view data being gathered by the detector) may be anywhere from about half the revolution time of the gantry to several revolutions of the gantry. The view data to produce images of this new slice region is gathered over substantially all of this traverse. All of this view data is gathered using scan parameters determined before the slice region enters the imaging volume, in most cases scan parameters chosen before the start of the helical scan. If the slice region contains structures previously unknown, for example, an unlocated lesion in a human patient or a hidden flaw in a manufactured object, the predetermined scan parameters may be adequate to detect the presence of the structure, but sub-optimal for giving detailed information on it. In some cases, the user of the CT scanner may decide to do a re-scan of the object using more optimal scan parameters after images from the first scan have been produced and viewed and the location and general nature of a particular region of the object is appreciated for the first time.
In U.S. Pat. No. 5,796,802, Gordon discloses systems and methods for using a pre-specified subset of CT imaging views, generally widely spaced in projection angle, to test for the presence of target structures in the imaging volume. Only if the test indicates sufficient probability of a target structure are CT images reconstructed, or is a complete scan made if one had not been theretofore. In U.S. Pat. No. 5,818,897 the same inventor discloses a two-dimensional CT detector system with one class of detector cells for imaging and a different class of detector cells for collecting view data to be separately tested to find predetermined target structures, in one embodiment by examination of sinogram data. While Gordon's patents contain methods which test view data to find target structures, in neither invention does he address the challenge of the preceding paragraph, namely determining the presence of target structures as soon as possible after their entry into the imaging volume of a helical scan so the imaging process can continue from that point with changed and optimized scan parameters.